Method of treating tin plate



g- 12, 1952 R. A. NEISH 2,606,866

METHOD OF TREATING TIN PLATE Filed Oct. 27, 1948 Inventor: 5/0/4919 4 A f/s/l',

thermore; to serve Patented Aug. 12 1952 mama s Tme s new 7 i Richard :A.-Neis.h, 'Piittsb ssh ita s s teel Company, a corporation of enl a e emer 12a@ a am sae V.

This invention relates to an improved method of treating tin-plate such asithat iised inmaking cans. The object of the inventionis to prodace tin-plate which is resistant to the formation of yellow stain during storage, to discoloration such as occurs during the heatfcuring o'f enamels, to

etching, and to sulfide staining such asccustom-arily occurs in the canhand is adapted, furas a suitable base for subsequentlacquering.

-Nu merous' methods have been proposed for treating tin-plate to improve its characteristics and inhibit the development of deiectslsuch as baking discoloration and sulphide 1 stain which usually show up only'after the'materialhas been used-for its ultimate purposesu'ch as lithograph ingstock or containers for food. Some of these methods have been successful in a measure but none of them has provided a complete solution of the problem. Stevenson Patent No. 2,524,713,

for example, discloses a method which increases resistance to sulphide discoloration but 'doesnot prevent baking discoloration. It is accordingly a--further object of myinveintion to providefa method superior to those previouslysu ggested, .in effectiveness; speed; and economy; j f Q l Yellow stain on tin plat'e is known to be the visible manifestation'of atmospheric oxidation of the'exposed surface offthe tin; j Both chemical and electrical oxidation (byanodizing) have'been recommended as means iorfminimizing' this de feet. Neither-one is "s'ufiiciently efiective alone, however, to prevent this discoloration, although a dense coating of anodic oxidegive's some protection and increases resistance to sulphide stain ing. Another prior method of treating .tin5plate makes use of the fact that tin ox desformcomplex compounds with many'pas'sivating radicals (chromates, phosphates, molybdates, etc.) which provide a surface layerinhibitingjtofa degree, further oxidation as well chemical attack: by dilute acids or alkalies. a treatment converts only t the outer surface of the original oxide, and the surface portion so. affected extremely thin andlcomparatively per,- meable so that the cushion of unconvertedoxide beneath the superficial protective layer'canfgcontinue to grow and eventually results in the'jappearance of yellow stain. Similarly andfor the same reasons, this superficial protectivelayer does not ofier comparable, resistance to sulfi'de. staining' or to chemical'attack, and is toufndto'be less resistant to mechanical abra s io'n' arising from operation of the treating and related ismsesses. "Even though cathodic electrolytic clean- The difficulty is that such ingbe resortedto heforefsuch an immersiontreat- "nient'fthe oxide remrmsiq'nm tifrisiiiface imfniediateIy the camoui'ciacncnfeeasesafiatiie net 1 iepsmcnpr LuncOniLertedoxide pen am "a thin superficial protective layrf" I' have discovered a ni'eth'odf for treating tinplate whereby the fini'ti "l i'oxid ,cbatiiig resulting rrqm atmospheric 7 a presence of a .passiva mg'i-adial. This insures that re oxidation will finder conditions causing the formation ofl'aii effective protective film of the desired" complex directly on'theitin without an intermediate layer of'ioxide. Gen:- erally speakingf my ,nithod involves .th cathodic c eaning of tin-plate "in an electrolyte including passivating Iradicals, follow'e'df immediately by anodizing the material "in t e sa ine electrolyte. 'Ifliis,causesired'uction' of lthe' oxideifilm initially pres nt on the material ,andv ermits the'film of the complex to "beitherflouilt up giii gi to thickness ,I l fiicient to inhibit subsequent oxidatioh or "th'tiri therebeflow." After f'extensive ex-- perimentl have fouhdthat chromates'or mixed chromates a'rid'jpho'sph'ates are most desirable as passivatingfradicals";allthings cohsidered.

' Mo r l f in iy niiel ce zs sts i assm'g ti'r'i'I-hlate .throu gha dilute solution conand phosphate 0 san" m ihgti e material first es e i' i e? was A a esult the oxide'fil'r'ninitiallygp se dri the matei ial i s first e ah hnii 1 s? i th pr sence o s m st radibalewith wh ch the oxide ombifles'wf xfiflil s les la oup conbl den e adheren and gp ot ct onaeain fu s... i

an \ru v i, s Wi fi e eiralallewed fiq s es sa e o o i ais9lui neent ein f om .2 t 2%"Tofialchroinate; e. g, .the chromfii e of an allga infetalfsuch i assodium qichliqmate W il -t t fll n9e i fei n..ma he varie considerably, decre sing it below ;1.5 reduces dl i i l. fibsi fi findi mlcsntrat cns e 3 1 i isla tiyel ne ie tiv .Convelsli mages the concentrat on, ab ve 2.9 doesjnot" result in a significant I improvement alh eh fishfly higher ,cdnqelilf tien ay 3.2%, 21% ?4 l @@Yi%bc fo ti e u poseo increa ng ithei w ment is necessary. Instead of the dichromate im te v be or eplenishacid, if necessary. I have produced resistant films at various acidities between pH 2 and 11; however,

a pH less than 4 does not improve results, while films formed at a pH greater than '7 .do not offer satisfactory adherence with certain types of tinplate lacquers.

The action of the treatment in forming a protective film appears to be independent of tem-" perature at least in the range between room temperature and 180 F.

The method of my invention may be carried out most efficiently by drawing a continuous strip of tin-plate, such as that produced by known electrolytic tinning processes, through the solution described above and past electrodes immersed therein with respect to which the strip is first made a 'cathodeand then an anode. The process may be convenientlypracticed by the aid of apparatus shown diagrammatically in the single figure of the accompanying drawing.

Referring in detail to the drawing, tinned steel strip is unwound from a coil and passed through a bath of the aforesaid solution contained in a tank ll provided with entry and exit guide rollers I2 and I3 and hold-down rollers l4. Spaced pairs of electrodes and iii are immersed in the bath, the electrodes of each pair being on opposite sides of the path of the strip. A current sourcesuch as a generator lTis' soconne'cted to the strip and electrodes [5 as to make the portion of the strip between them the cathode. A similar source H3 is so connected to the strip and electrodes I6 as to make the portion of the strip between them the anode. Connections to the strip are shown as made through rollers l2 and I3 which must therefore be conducting and insulated from the tank if it is also conducting but any other suitable forn'i-ofconnection to the strip may be used. After emerging from the solution in tank 1 I, the strip passes through'rinsing and drying apparatus indicated at 19 and 20, respectively, and is then wound-up by a 'recoiler 2! which exerts the tension necessary to pull the strip through the lineof processing apparatus.

The strip may be processed at speedsof several hundredfeet per minute, since the treatment requires only a very short time. Operating at 450 feet per minute, for example, with electrode 5' long, gives good results although the time of exposure of a given point on the strip to cathodizing is only .6 or .7 second and the same for anodizing. The current density required for the cathode treatment will -vary with the degree of initial oxidation of tin surface, and with the allowable time interval, but 100 amperes per square foot is about the maximum needed, while a much lower density, such as 30 amperes, has been found satisfactory in some instances' The thickness and characteristics of the films obtained will depend upon the anodizing process; that is, they are a function of the time and the current density. I have produced films by using anodic current densities ranging from one to 90 amperes per square foot. However, those formed witha current density greater than 30 amperes per square adherence of phenolic tin-plate lacquer.

by capillary rise.

4 foot tend to be more porous in nature, are less protective, and are not generally suitable as lacquer bases. Film produced with an anodic current density of 10 to 30 amperes per square foot are protective and suitable for certain applications, although the adherence of phenolic type lacquers may be poor. The most effective films for all purposes, however, are produced with a current density of 1 to 10 amperes and more specifically 4 to 7 amperes.

, Tin-plat processed according to my invention exhibits improved characteristics in several respects, in comparison with material treated by methods known previously. In the first place, it

has less tendency to discoloration and sulphide staining as well as good resistance to etching in dilute acid. It.is further characterized by good This adherence is better, the lighter the layer of protective film laid down during the anodic treatment. The product of my method also shows better solder-flow characteristics as measured Furthermore, the adhesional strength of the soldered areas is not adversely affected.

Most oxide films have little resistance to abrasion, but material treated according to my invention acquires an oxide coating having greater resistance than the products of known methods. In fact, the protective film produced by my method is comparatively unaffected by abrasion on passing through the apparatus comprising the processing line. I

My. improved method produces a uniform protective film which'may be easily controlled as to thickness. Use of the phosphate radical produces a surface of pleasing appearance which is nontoxic, tasteless and odorless. The process may be carried on at high speed in a continuous production line, since, the film formation requires only a second or less with suitable current density. Thecost is low because of the high speed which is practical'andthe cheapness of the salts employed; There is no factor in the control of the method which is unduly critical. The process utilizes the electrical resistance of the protective film to insurefuniform and'complete coverage. That is to. say,'when th film has been formed on a'givein area of the tin-plate, no further oxidatio'rioccur's therein because of-the electrical resistance of the film, until all other portions of the material in the bath have been coated to the samethicknss. The rate of film formation is a maximum at the beginning thus insuring immediate protection at all points on the surface of the material.

While the specific example of the method described in detail herinabove involves the processing of tinned strip, the invention is also applicable to the treatment of tinned sheets such as produced by the hot-dip'method of coating tinplate.

Althoughl have described only a preferred practice of the invention with certain modifications, it will be understood that changes in the procedure disclosed may be made without departing 'from the spirit of the invention or the scope of the appended claimsr Y Iclaim:

1.. Infa'method of treating sheet material havinga tin'surface, to inhibit the tendency thereof to sulphide staining and baking discoloration and increase thev adherence of lacquer, the steps including firstmaking the material cathode in anacid solution containing from .2 to 3% of a REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS' Number 10 Number Name Date Sumner Sept. 17, 1940 Cook et al Feb. 23, 1943 Stevenson July 29, 1947 Glock Oct. 5, 1948 Prust Apr. 4, 1950 FOREIGN PATENTS Country Date Great Britain Mar. 21, 1934 Great Britain June 9. 1930 

1. IN A METHOD OF TREATING SHEET MATERIAL HAVING A TIN SURFACE, TO INHIBIT THE TENDENCY THEREOF TO SULPHIDE STAINING AND BAKING DISCOLORATION AND INCREASE THE ADHERENCE OF LACQUER, THE STEPS INCLUDING FIRST MAKING THE MATERIAL CATHODE IN AN ACID SOLUTION CONTAINING FROM 2 TO 3% OF A CHROMATE OF AN ALKALI METAL AND HAVING A PH VALUE OF FROM 2 TO 7, THEREBY AT LEAST PARTIALLY REDUCING THE OXIDE FILM INITIALLY EXISTING ON THE MATERIAL, AND IMMEDIATELY THEREAFTER MAKING THE MATERIAL ANODE WHILE STILL IN SAID SOLUTION, THEREBY RE-OXIDIZING THE TIN SURFACE AND FORMING THEREON A WATER-INSOLUBLE PROTECTIVE FILM OF A COMPLEX COMBINATION OF THE OXIDE AND SAID CHROMATE. 